The present invention relates to a thermal transfer sheet and a thermal transfer image forming method, more particularly to a thermal transfer sheet and a thermal transfer image forming method for providing an image of high quality with little color difference in the gradation of an image using sublimable dyes (thermally migrating dyes) in the method thereof.
In recent years, there have been developed some printing methods for providing excellent mono-color or full-color images conveniently at high speed such as an ink jet method and a thermal transfer method replacing conventional ordinary offset printing and other printing methods. Above them, a so-called sublimation thermal transfer method using sublimable dyes is most distinguished, which can provide a full-color image superior in the gradation characteristic as excellently as a color photography.
In this sublimation thermal transfer method is usually used a thermal transfer sheet comprising a substrate film such as a polyester film, a dye layer containing a sublimable dye and a binder formed on one surface side of the substrate and a heat resistant layer formed on the other surface side of the substrate for preventing a thermal print head sticking thereto.
The thermal transfer sheet is superposed on an image-receiving sheet having a dye receptor layer made of e.g. polyester resin with the dye layer of the former sheet facing the latter sheet and imagewise heated from the back surface side of the former sheet by means of the thermal print head, which causes the dye of the dye layer to transfer to the image receiving sheet to thereby form a desired image.
In the above thermal transfer method, when a color image is formed by three or four colors, colors except yellow, magenta and cyan are reproduced by way subtractive mixture of these three primary colors. Such medium colors and black are sufficiently reproduced by suitable selection and combination of respective dyes of yellow, magenta and cyan; for example, black is reproduced by blending these three colors. However there rises a problem that black, sufficiently reproduced in a shadow area, is often reproduced rather reddish or bluish in a highlight area. The reason is that the arrangement of the three colors is determined for black to be most sufficiently reproduced in a shadow area. Reversely, if the arrangement of three colors is determined for black to be excellently reproduced in a highlight area, black in the shadow area cannot be sufficiently reproduced. Such problem occurs likewise in cases of other medium colors such as green, red and blue, which is an obstacle in color reproduction process for forming images superior in the gradation characteristic.
Another problem is that, where two or more dyes are overlapped to reproduce a subtle tone color using prior thermal transfer sheets of yellow, magenta and cyan, the printing energy control is required for each color of them according to each level of density thereof. A conventional printer, however, does not have a capacity enough to store and process such enormous data for controlling the printing energy. On the other hand, if the data is simplified so as to be processed even by the conventional printer, it stands to reason that a subtle tone color cannot be well reproduced.